1. Field of the Invention
This application relates to detection of explosives, and more particularly, to calibrating the ion mobility drift time scale of an ion mobility spectrometer for the detection of explosives.
2. Description of Related Art
Ion mobility spectrometers (IMS) are used for the identification of chemicals based on the time required for the chemical ion to traverse a drift space to a charge collecting surface under the influence of an electric field. The resultant drift time is dependent on the field strength, the distance traversed, the density, type, and flow vector of the gas within the drift space, and the physical characteristics of size and mass of the chemical species. In practice, usually the gas within the drift space is temperature-controlled dry air, and the drift distance, electric field strength, and electric field uniformity are fixed or controlled. This leaves ambient pressure, subtle irregularities in temperature, and ambient chemical species as the primary uncontrolled variables affecting the drift time. The physical properties of the chemical species together with that of the drift gas are combined into a single parameter known as the reduced mobility constant, usually written as Ko. In general, a large value of reduced mobility equates to rapid motion of an ion in an electric field. The reduced mobility is the mobility corresponding to standard conditions of temperature and pressure, 273 Kelvin and 760 mm.
In order to further compensate for subtle variations in temperature and pressure, a calibrant chemical may be introduced into the ion generating region of the system. Since this chemical is known, the expected drift time can be estimated if the ambient pressure is measured. For a system that is open to the atmosphere, it is sufficient to measure the pressure external to the ion drift region. The actual calibrant drift time may then be used to predict the observed drift times of any other desired ion species by reference to a table of the ratio of the reduced ion mobility of the calibrant to the target chemical. Relying on the assumption that the environment seen by the calibrant ion and the target ion is similar through the ion source and drift region, any imperfections, non-uniform electric fields, temperature variations, and the like are equally compensated for in the calibrant and target ion drift times, and the ratio should be a constant.
One variable that is species-dependent is the level of ambient humidity in the drift region. It is well-known that water molecules form clusters that attach metastabily to certain chemical species, thus changing their mass and cross section, and thus ion mobility in the drift region. In order to minimize this phenomenon, the air within the drift region is typically both heated in excess of 100 degrees Centigrade as well as dried using a drying agent, such as molecular sieve material.
In a system that is open to the atmosphere, the calibrant chemical is typically admitted along the same path that is employed for the external air sample entry into the ion generation region. Therefore, the calibrant encounters potentially humid external air and may be compromised in some manner by its presence.
In practice, the charge peak of the calibrant chemical is first identified in the spectrum of charge amplitude versus drift time after the calibrant has been introduced into the ion mobility spectrometer. The specific drift time is determined, and the expected drift times of target chemicals are calculated from a pre-measured table of the target chemical/calibrant ratios of reduced mobilities.
A “window” is generally provided around the expected drift times of the target chemicals because of effects related to instantaneous drift gas pressure and flow speed, metastable chemical adducts, gas concentration, as well as many other subtle effects.
A commonly employed calibrant is butylated hydroxytoluene (BHT), which is a food additive normally used for its anti-oxidant properties. This chemical is fairly sensitive to the level of ambient humidity and contamination chemicals in the environment. It is characterized by two chemical constituents in the gas phase, BHT and BHT+2O. These species have ion mobilities with almost, but not the same, value. The resultant charge peak in the ion mobility spectrum is thus the sum of two Gaussian-like peaks, which is in turn also a Gaussian-like peak. Each of the two chemical species has an ion transmission amplitude that varies independently with humidity and the presence of contamination chemicals from the environment. When the relative amplitudes of the two peaks vary, the apparent drift time of the sum of the two peaks will shift over the range between the mobility of each of the constituent species. Common contamination chemicals, including water vapor, are known that almost completely block ion formation of BHT ions. As such, its unstable ion transmission amplitude and variable drift time means that BHT is not a reliable calibrant for a system that is open to the atmosphere.
Accordingly, it would be desirable to provide a system and method using a calibrant chemical having a drift time and transmission amplitude that are only weakly affected by ambient environmental chemicals, including humidity, in order to ensure the stability of the calibration of the IMS.